I occasionally hear earth's atmosphere and/or oceans described as a "heat engine" pumping air and water and energy around.
I think this is highly questionable nomenclature. I'd like a second (or third or tenth) opinion.
A heat engine takes an amount of thermal energy from a hot reservoir, Q(h), converts some of it into work that is extracted from the engine, W, and then gives remaining energy to the cold reservoir, Q(c).
Q(h) - Q(c) = W.
No matter how I look at it, there is no work done by the atmosphere and/or oceans. On the largest scale, the hot reservoir is the sun and the cold reservoir is space. These two (approximately) balance. And the imbalance is due to global warming/cooling, not to any work being done. Sure, the temperature differences drive winds and ocean currents, but these are *internal* to the 'device' and no net work is extracted. Whatever subsystem I consider, I can still see no work being extracted.
I suppose you could call it a heat engine with efficiency = 0, but then you would have to call any system with convection a 'heat engine'. Heck, even conduction or radiation from hot to cold would be a 'heat engine'.
I am fine in colloquial settings letting people use the term 'heat engine' as a short hand for "it moves because of heat", but there is no 'heat engine' in the technical physics/engineering sense.